t0090: mark add-interactive test with PERL prerequisite

The add-interactive system is built in perl. If you build
with NO_PERL, running "git commit --interactive" will exit
with an error and the test will fail.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0090-cache-tree.sh

@@ -131,7 +131,7 @@ test_expect_success 'second commit has cache-tree' '
 	test_cache_tree
 '
 
-test_expect_success 'commit --interactive gives cache-tree on partial commit' '
+test_expect_success PERL 'commit --interactive gives cache-tree on partial commit' '
 	cat <<-\EOT >foo.c &&
 	int foo()
 	{